The present invention relates to an improved wellhead valve which includes a back pressure valve and which is installed in a wellhead hanger during the removal of the blowout preventer and the installation of the Christmas tree. Prior valves of the type contemplated herein have included a body which can be secured within the hanger, such as by threading it into internal hanger threads, a seal for sealing against the interior of the hanger and a bore through the body with a valve seat facing downwardly and surrounding the lower exit of the bore into a counter-bore. A valve member is mounted within the counterbore and is biased by a spring into seating engagement with the valve seat. Also, any pressure which exists in the well below the valve is exerted against the valve member to cause it to maintain its sealing engagement with the valve seat. Flow is possible through the valve from the interior of the hanger above the valve if sufficient pressure is provided to overcome the spring force and any pressure force urging the valve closed. An example of this type of valve is shown in U.S. Pat. No. 2,939,534 to H. Allen.
Another example of a prior art structure is shown in U.S. Pat. No. 4,058,162 to J. Smith which discloses a plug for sealing within a hanger and is secured therein by the camming action of an internal sleeve which cams the locking elements into an internal groove within the hanger. This structure also includes an internal valve seat and a valve which is adapted to be closed by pressure from below.
The W. Boyle U.S. Pat. No. 3,250,331 discloses a plug for use in a well flow passage and includes a valve seat, a valve member biased to engage the valve seat and close flow from below the plug and a special garter or coil spring lock to engage within a groove in the hanger or other well structure in which the plug is to be positioned.
A two-way check valve is used when the tree above the hanger is to be pressure tested. This valve prevents flow in both directions and allows pressure testing of the tree without having leakage through the valve or having to bias the valve sufficiently to prevent such leakage. Examples of the use of such check valve in place of the back pressure valve is discloses on pages 1366 and 1367 of the 1978-79 Composite Catalog of Oil Field Equipment & Services and on pages 1712 and 1713 of the 1982-83 Composite Catalog of Oil Field Equipment & Services.
As mentioned above there are times when it is desired that the back pressure valve provide a seal against pressure from both directions. One occasion is the decompletion of the well. This occurs when the tree is removed and a blowout preventer installed in its place, thus reversing the normal completion procedures.